- Published: 02 February 2010
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Edward Hilliard, age 12, of Allentown, Penna., for his question;
What is a comet?
Here you disappointed in the little comet that visited our skies this spring? According to reports from old timers, a comet should be a showy fellow with a golden tail streaming over the sky ‑ bright enough to be seen at high noon. We had to search the skies to find our modest visitor, But Andy says you can cheer up, chances are, you will yet see one of the really big show‑offs. For almost everybody lives to witness a visit from at least one major comet.
Quite a few dim comets visit our skies every year. Most of them show up only in telescopes and telescope pictures. Once in while, the astronomers tell us of one large enough for our eyes to see ‑ if we look in the right part of the sky. The really big comets are rare, once or twice in a lifetime spectacles. They put on such a show that no one needs to point them out. They can be seen for weeks at a time, often in the daytime as well as at night.
Comets are members of our suns family and not visitors from outer space. Like the planets, they travel a scheduled orbit around the sun. The planets travel in rough circles, keeping a fairly even distance from the sun. Not so the comets. Their orbits are long, thin ovals. The sun is just inside one narrow end of such an orbit. The other end is a U‑turn, way out in space, maybe 3,000 million miles from the sun at the other end.
Some comets take hundreds of years to travel once around their orbits, some only a few years. Most of the trip is a dull, drab journey through space. A comet comes to life only when it swings around close to the sun. Only then do we see it in all its glory. The big shiny head looks like a brilliant star trailing its golden hair over the sky.
Of course, a comet is not a star. Itis a bundle of dark, dead solid material. Like our planets, it shines only with the reflected glory of the sun. A comet's head is often far, far bigger than the earth. But sometimes the distant stars can be seen right through it. Experts think the head maybe a bundle of loose stones and meteors. In any case, the earth weighs much more than many thousands of big comets.
The comet wears its golden train only when it visits his majesty, the sun. The pressure of the sun's rays is believed to force dust and gases back from the visitor. Hence, the golden trail always streams away from the sun. As the comet makes its U‑turn around the sun, it sometimes goes close enough to pass through the outer corona gases of. the sun. At this point the comet is traveling at a terrific speed. Yet the tail swings around still faster, always streaming away from the sun.
When the U‑turn is finished, the comet is moving away from the sun and the tail streams ahead of it. The light fades as distance grows between the comet and the sun, the tail disappears and soon the bright head of the comet is a dim blur. Soon it will be no more than a loose bundle of dark stones chugging wearily around its lonely orbit between the planets.